Method for producing crosslinked acrylate hot-melt adhesive compounds

ABSTRACT

The invention relates to a method for producing a polyacrylate adhesive compound by a hot-melt method. The inventive method is characterized in that a polyfunctional α-splitting agent which is present as an oligomer is added to the polymer to be crosslinked prior to the hot-melt method and UV crosslinking is carried out after treatment in the hot-melt method.

[0001] Process for preparing crosslinked polyacrylate pressure-sensitiveadhesives via a hotmelt process with subsequent crosslinking by means ofultraviolet radiation.

[0002] Polyacrylates—especially pressure-sensitiveadhesives—crosslinkable by UV radiation have already been known for along time. They offer a variety of advantages over the conventional,thermally crosslinkable systems. For instance, UV-crosslinkable acrylatepolymers, applied two-dimensionally to a backing from solution, forexample, can be crosslinked in the desired way by varying the activatorconcentration and the UV dose. Controlling the crosslinking density andhence the properties of the product is therefore easy to do, in additionto the dosing, by way of the operational parameters—in this case, the UVdose. Polymer systems which can be dynamically controlled in this waysignify a substantial advantage in a modern production structure, when acomplex product portfolio is to be realized starting from a few basicbuilding blocks.

[0003] UV crosslinking can be achieved by different methods. In thesimplest case, a UV activator is added to a customary, saturatedpolyacrylate. A distinction is made here between type I and IIactivators: the former cleave following UV activation, the latterabstract hydrogen atoms after UV activation.

[0004] It has been found that admixed type I photoinitiators aregenerally incapable of generating crosslinkable free radicals on acustomary saturated polyacrylate. Other, competing reactionspredominate. If a type II photoinitiator is added, in contrast, acrosslinking reaction can be achieved for a polyacrylate compositionwithout further modification, in the case, for example, of a copolymerof 2-ethylhexyl acrylate and acrylic acid. This reaction, however, isnot very efficient if resins are admixed in order to increase the bondstrength. A particular reason for this is that the resins absorb UVlight, have a strongly regulating influence, and so adversely affect oreven prevent UV crosslinking. Even the addition of polyfunctionalacrylates or methacrylates for increasing the crosslinking efficiencyhas little effect. Furthermore, unreacted acrylate groups may lead to adisruptive post-crosslinking, which is disadvantageous for the stabilityof the product properties.

[0005] A more elegant method lies in incorporating vinylic double bondsinto the acrylate polymer from the outset in order to facilitatecrosslinking.

[0006] U.S. Pat. No. 4,234,662 describes a process for preparing hotmeltadhesives having pressure-sensitive properties which consists of thefollowing steps:

[0007] Copolymerization of allyl acrylates or methacrylates with atleast one copolymerizable acrylate monomer to give a prepolymer which issolid at room temperature; heating of the resultant solid allylicallyunsaturated prepolymer to a temperature at which it is liquid and fluid,and coating of the liquid prepolymer onto a substrate. The coatedsubstrate is subjected to electron beams of from 1 to 4 megarads capableof crosslinking the polymer to form a cured, pressure-sensitiveadhesive. A disadvantage of this process is the difficulty of itsimplementation, since during the free-radically initiated polymerizationand during coating from the melt the polymers tend to form gel.

[0008] U.S. Pat. No. 5,391,406 and U.S. Pat. No. 5,416,127 thereforepropose targetedly incorporating the polymer-bonded vinyl groups bymeans of a polymer-analogous reaction withdimethyl-meta-isopropylenyl-benzyl isocyanate (m-TMI). Polymersfunctionalized in this way can be coated from the melt without forminggel, since the ceiling temperature of the thermal polymerization of theisopropenyl group is below the coating temperature. If a photoinitiatoris added to the polymer, the composition is UV-crosslinkable.Nevertheless, this method too has its practical disadvantages: on theone hand, the operation conducted, by way of the polymer-analogousreaction, is very laborious and therefore inconvenient; on the otherhand, high molecular mass, double bond functionalized polymers such asare produced in this case are particularly susceptible to shearing athigh temperatures and therefore result in severe unwanted gelling in thecourse of processing by the hotmelt process, where high shearing forcesact on account of the extruder processing operation.

[0009] Copolymerizable photoinitiators also make crosslinking easier.For instance DE 24 43 414 A1 describes a process for producing productsmade self-adhesive using an adhesive based on crosslinked polyacrylicacid derivatives, in which the adhesive contains in copolymerized formfrom 0.01 to 5% of an acrylic or methacrylic acid(2-alkoxy-2-phenyl-2-benzoyl)ethyl ester photoinitiator and iscrosslinked by brief irradiation with ultraviolet light.

[0010] The (meth)acrylic esters used therein as polymerizablephotosensitizers however, are obtainable only by a multistage synthesiswith a relatively poor yield and, furthermore, are not very efficient inUV crosslinking.

[0011] For the purposes of improvement, DE 27 43 979 A1 describes aprocess for producing products coated with a self-adhesive compositionbased on crosslinked polyacrylic acid derivatives by copolymerization ofa photoinitiator into the adhesive and subsequent crosslinking on thebacking material by means of short-term UV irradiation, wherein 0.01 to5% by weight of benizoin acrylate or benzoin methacrylate photoinitiatorare copolymerized into the self-adhesive composition.

[0012] The adhesives described in these patents are preferably preparedby polymerization in solution and then coated from the solution onto abacking. Only the dry film can then be subjected to UV irradiation.Generally, however, the copolymerized photoinitiators have a drawback:the preparation of a UV-crosslinking acrylate pressure-sensitiveadhesive system is very limited by the fact that only a very fewphotoinitiators are available as copolymerizable compounds without greatinconvenience and therefore in a way which is of interest economically.

[0013] EP 0 578 151 B1 describes the use of pressure-sensitive adhesivesbased on polyacrylate with copolymerized benzoin derivatives for thecontinuous industrial melt coating of medical products with subsequentcrosslinking of the adhesive by UV irradiation. Here again, theabovementioned restrictions apply in relation to the availability ofphotoinitiators of economic interest.

[0014] BASF AG describes copolymerizable benzophenone derivatives whichfollowing UV irradiation are able to abstract hydrogen atoms (EP 0 343467; U.S. Pat. No. 5,047,443; U.S. Pat. No. 5,264,533). Unlike thecleavable activators, these photoactivators do not form low molecularmass constituents which can evaporate from the cured coat of compositionand adversely affect its properties. For instance, the UV irradiation ofthe systems, which are described in, inter alia, the patent DE 27 43 979A1, is followed by the formation of benzaldehyde, which can be perceivedas a disruptive odor.

[0015] All of the photoinitiators hitherto described and/or theUV-crosslinkable acrylate prepolymers prepared from them are subject toa marked restriction in terms of the wavelength range which possesses aninitiating effect with the crosslinking reaction. On account of itsabsorption maximum, every photoinitiator incorporated into the system bycopolymerization determines the wavelength range subsequently availablefor crosslinking.

[0016] This produces a limitation if this acrylate prepolymer is to becrosslinked by means of ultraviolet radiation in the form, for example,of a relatively thick coat, a resin-blended polymer composition or apigmented composition. Thus the multiplicity of the conventional systemscan be crosslinked sufficiently only up to a limited coat thickness (forexample, the system from the abovementioned DE 27 43 979 A1 up to a coatthickness of 50 μm). For thicker coats, the polymer no longer possessessufficient UV permeability in a wavelength range within which thephotoinitiator is active. Pigmented coats, for example, those of polymercompositions colored white by titanium dioxide, can no longer beUV-crosslinked at all in a wavelength range of 250 nm (absorptionmaximum of the abovementioned benzoin acrylates).

[0017] U.S. Pat. No. 5,194,455 describes a process for preparingacrylate-based pressure-sensitive hotmelt adhesives by irradiation of asubstrate for which at least one acrylic monomer containingN-tert-butylacrylamide (TBA) is copolymerized, it being possible for thereaction mixture for polymerization to further comprise one or moreethylenically unsaturated compounds which are not part of the group ofthe abovementioned acrylic monomers, the resultant solid polymer isheated and brought in a fluid or liquid state onto a substrate, it beingpossible for additives and auxiliaries to be present, and the coatedsubstrate is irradiated with high-energy radiation.

[0018] WO 96135725 claims a crosslinked, pigmented, pressure-sensitiveadhesive which comprises a copolymer which is coated onto a substrateand cured by UV radiation such that it possesses a shear resistance ofat least 20 hours at 70° C., said copolymer comprising a pigment, ahydrogen-abstracting photoinitiator, and an acrylate-based copolymerprepared from a substantially acid-free monomer mixture whosecomposition is as follows:

[0019] (i) about 30 to 99.9% by weight of at least one alkyl acrylate,the alkyl group possessing 1 to 24 carbon atoms,

[0020] (ii) about 0.01 to 15% by weight of a monomer containing atertiary amine group,

[0021] (iii) 0 to 25% by weight of at least one polar vinyl monomer, and

[0022] (iv) 0 to 30% by weight of a diester of an ethylenicallyunsaturated dicarboxylic acid,

[0023] based in each case on the total weight of the monomer.

[0024] The crosslinking of coats of these polyacrylates requires longwave UV activators, such as isopropylthioxanthone. A disadvantage ofcopolymerized dimethylaminoalkyl compounds is the restriction tocomonomers free of carboxylic acid, as occur in all implementations inWO 96/35725. For example, the thermally initiated solutionpolymerization of acrylate monomers, acrylic acid, and olefinicallyunsaturated dimethylaminoalkyl compounds leads after a short reactiontime to a sharp rise in viscosity and to gelling.

[0025] Polar acrylate polymers in particular, and especially thosecontaining carboxylic acid, are particularly advantageous forhigh-performance adhesives. Internal strength, which is significantlyinfluenced by interpolymeric hydrogen bonds, can be assisted mostefficiently, in addition to the actual crosslinking, by means ofcopolymerized acrylic acid. In addition, the peel increase known forpolyacrylate, by means of which an increase in the bond strength isachieved, is substantially improved in the presence of significantproportions of copolymerized acrylic acid.

[0026] For saturated systems in particular the customary initiators areunsuitable or are suitable only when the system to be crosslinked isactivated by means of comonomers or of adjacent electron donor groups.As a result, there has to date been no description of a UV-crosslinkingsystem which is capable of effecting efficient crosslinking ofpolyacrylates without functional comonomers.

[0027] A disadvantage of the photoinitiators which are generallyemployed, furthermore, is that for the multiplicity of these initiatorsthey do not withstand the hotmelt process; in other words, that theylose their reactivity as crosslinking initiators on heating totemperatures which are customary in such processes.

[0028] Benzoin derivatives and benzil derivatives are not suitable forhigh reaction temperatures. Thus “Chemistry & technology of UV & EBFormulation For Coatings, Inks and Paints, Volume 5, PKT Oldering, 1994”mentions benzoin methyl ether and benzil dimethyl ketal as examples oftype I photoinitiators which are frequently employed. Both thesephotoinitiators cannot be used, or can be used only with greatrestrictions, for the hotmelt process, since they are unstable over aprolonged period of time and undergo decomposition. This is particularlythe case in systems containing acrylic acid, which generally form thebasis in the case of polyacrylate pressure-sensitive adhesives.

[0029] Although benzophenone as a hydrogen-abstracting initiator (typeII) has a higher thermal stability, it undergoes a high degree ofsublimation under thermal load and under vacuum from the acrylatepressure-sensitive adhesive. Similarly behavior is shown bycamphorquinone, which likewise has only a relatively low molecularweight and therefore tends under vacuum at elevated temperatures toundergo volatilization from the composition to be crosslinked.

[0030] It is an object of the invention to offer a process for preparinga polyacrylate pressure-sensitive adhesive in which a crosslinkablesaturated polyacrylate system is processed by the hotmelt process andcan then be crosslinked. This system ought to be stable to the drasticconditions of the hotmelt process over a relatively long period of time;that is, it ought to have a correspondingly high pot life. Gelling ofthe polyacrylate system during the hotmelt process should be avoided, atleast to a large extent. The aim here is to find a method ofcrosslinking which does not require the presence of additionalcomponents or groups which activate crosslinking. The operation ofcrosslinking should be unaffected, or not substantially affected, by thepresence of additives in the pressure-sensitive adhesive.

[0031] This object is achieved, surprisingly and unforeseeable for theskilled worker, by a process as described in the main claim. Thedependent claims relate to advantageous variants of this process. Alsoclaimed is the use of a polyfunctional α-cleaver present in oligomericform.

[0032] The main claim accordingly provides a process for preparing apolyacrylate pressure-sensitive adhesive by way of a hotmelt process inwhich a polyfunctional α-cleaver present in oligomeric form is added tothe polymer to be crosslinked, prior to processing by the hotmeltprocess, and the UV crosslinking is carried out after processing by thehotmelt process.

[0033] Compounds termed α-cleavers are those which function asinitiators and which during the initiation step, as a result for exampleof ultraviolet irradiation, undergo intermolecular cleavage and in doingso break down into free-radical fragments, with the bond to theheteroatom or carbon atom positioned α to a group which activates theformation of free radicals, in particular to a carbonyl group, beingcleaved.

[0034] Great preference is given to the use as α-cleaving initiator ofan α-hydroxy ketone for the purposes of the present invention.

[0035] In one particularly advantageous embodiment of the inventiveprocess the α-hydroxy ketone used isoligo[2-hydroxy-2-methyl-1-[4-(1-methylvinyl)phenyl]propanone]:

[0036] It is advantageous if the α-cleaver is used at from 0.1 to 5% byweight, in particular from 0.25 to 1% by weight, based in each case onthe polymer to be crosslinked.

[0037] Critical to the inventive process is that the α-cleaver used asphotoinitiator, in preferred form the α-hydroxy ketone, is notcopolymerized into the prepolymer but is instead admixed as a separatecomponent after the polymerization but before the crosslinking. Inonline operation this is synonymous with an addition before the hotmeltprocess, since crosslinking immediately follows this.

[0038] The UV initiators used are known in principle for this functionfor acrylate compositions. What was not to be expected was that theseinitiators would withstand the conditions of the hotmelt process over arelatively long time: thus, surprisingly, over a long period of time,neither decomposition nor other destruction was found, nor did theseinitiators undergo volatilization, as is observed generally no laterthan during the concentration process in the case of the UV initiatorscommonly used for the hotmelt process.

[0039] In order to increase the lifetime of the pressure-sensitiveadhesives and their precursors, they should be stored and/or processedin the dark.

[0040] In another very advantageous embodiment of the inventive processthe polymer to be crosslinked is prepared using any monomer mixturecomprising at least the following components:

[0041] a) 65 to 100% by weight of (meth)acrylic acid and (meth)acrylicacid derivatives of the general formula

[0042] where R₁═H or CH₃ and R₂ is an alkyl chain having 2 to 20 carbonatoms,

[0043] b) 0 to 35% by weight of vinyl compounds containing functionalgroups, specifically such that the sum of all of the components used is100% by weight.

[0044] Furthermore, it is a very great advantage if the polymer to becrosslinked has added to it 0.5 to 40% by weight of one or more resinsand/or 0.1 to 0.3% by weight of one or more—especially oligomeric—amineacrylates, based in each case on the prepolymer.

[0045] In one procedure which is advantageous for the process the UVcrosslinking is brought about by means of brief ultraviolet irradiationwithin a wavelength range from 200 to 400 nm, in particular usinghigh-pressure or medium-pressure mercury lamps with an output of from 80to 200 W/cm.

[0046] Also claimed is the use of a polyfunctional α-cleaver inoligomeric form, in particular an α-hydroxy ketone in said form,especiallyoligo[2-hydroxy-2-methyl-1-[4-(1-methylvinyl)phenyl]propanone], as UVinitiator for the crosslinking of polyacrylate pressure-sensitiveadhesives passing through the hotmelt process.

[0047] In addition to the polyacrylates set out above, crosslinking bythe inventive process can be carried out using all acrylatepressure-sensitive adhesives, especially those which possesspressure-sensitive adhesion properties in accordance with the “Handbookof Pressure Sensitive Adhesive Technology” by Donatas Satas (vanNostrand, N.Y., 1989).

[0048] The polymer to be crosslinked is prepared preferably by way of afree or controlled radical polymerization. The polymerization can becarried out in polymerization reactors, which are generally providedwith a stirrer, two or more feed vessels, reflux condenser, heating, andcooling, and are equipped for operating under N₂ atmosphere andsuperatmospheric pressure.

[0049] The free-radical polymerization is conducted in the presence ofone or more organic solvents and/or in the presence of water or withoutsolvent. The aim is to minimize the amount of solvent used. Depending onconversion and temperature, the polymerization time is between 6 and 48hours. The average molecular weight of the polymers varies between300,000 and 2,000,000 g/mol, preferably between 600,000 and 1,200,000g/mol.

[0050] Solvents used for the solution polymerization are preferablyesters of saturated carboxylic acids (such as ethyl acetate), aliphatichydrocarbons (such as n-hexane or n-heptane), ketones (such as acetoneor methyl ethyl ketone), special boiling point spirit or mixtures ofthese solvents. Great preference is given to using a solvent mixture ofacetone and isopropanol in which the isopropanol content is between 1and 10 percent by weight. Polymerization initiators used are customaryfree-radical-forming compounds, such as peroxides and azo compounds, forexample. Initiator mixtures can also be used. During the polymerizationit is also possible to employ thiols as further regulators for loweringthe molecular weight and reducing the polydispersity. Further compoundswhich can be used, referred to as polymerization regulators, include,for example, alcohols and ethers.

[0051] Advantageously, tackifying resins can be added to the polymers,with a percentage fraction of up to 40% by weight. For this purpose,without exception, it is possible to use all existing andliterature-described tackifier resins. As representatives, mention maybe made of pinene resins, indene resins, and rosins, theirdisproportionated, hydrogenated, polymerized, esterified derivatives andsalts, aliphatic and aromatic hydrocarbon resins, terpene resins andterpene-phenolic resins, and also C₅, C₉, and other hydrocarbon resins.Any desired combinations of these and further resins may be used inorder to adjust the properties of the resultant adhesive in accordancewith what is desired. Express reference may be made to the depiction ofthe state of knowledge in the “Handbook of Pressure Sensitive AdhesiveTechnology” by Donatas Satas (van Nostrand, N.Y., 1989).

[0052] The addition of amine acrylates is very advantageous, especiallythose present in oligomeric form, to the polymer to be crosslinked. Therate of crosslinking can be increased by this means. Products which havebeen found particularly appropriate are those offered in the trade nameGenomer™ from the company Rahn: Genomer 5248™, Genomer 5275™, andGenomer 5292™. In connection with the addition of the amine acrylates itshould be borne in mind that an increase in the rate of crosslinking isaccompanied by a fall in stability under hotmelt conditions.Advantageously, therefore, these compounds are not added until arelatively short time before crosslinking.

[0053] One advantage through the addition of the accelerators arises asa result of an expanded selection of the acrylate comonomers, which areno longer subject to the restrictions which hitherto applied and whicharose from the prior art. Thus, for example, it is also possible tocopolymerize into the polymer acrylate monomers containing carboxylicacid; here too, efficient crosslinking is possible and allowshigh-shear-strength, strongly adhesive acrylate pressure-sensitiveadhesives to be realized.

[0054] In order to increase the crosslinking efficiency, theuncrosslinked polymers are optionally blended with crosslinkers:suitable crosslinker substances for this purpose are, for example,difunctional or polyfunctional acrylates. However, it is also possiblehere to use all other difunctional or polyfunctional compounds which arefamiliar to the skilled worker and are capable of crosslinkingpolyacrylates.

[0055] The uncrosslinked polymers blended in this way are appliedpreferably as a hotmelt to a backing (PP, BOPP, PET, nonwoven, PVC,polyester, foam, etc.) or release paper (glassine, HDPE, LDPE),application taking place directly or by transfer lamination. UVcrosslinking then takes place advantageously directly on this backing.Where the amine acrylates described above are added, this can be done,in an outstanding form, immediately prior to coating, in order tominimize the time for which these compounds are exposed under drasticreaction conditions.

[0056] The acrylate pressure-sensitive adhesives prepared by theinventive processes may additionally be blended with one or moreadditives such as (primary and secondary) aging inhibitors, lightstabilizers, and ozone protectants.

[0057] In addition they may be filled with one or more fillers such asfibers, carbon black, zinc oxide, titanium dioxide, solid microbeads,silica, silicates, and chalk, the addition of blocking-free-isocyanatesalso being possible.

[0058] For documentation, the series of experiments set out below wereperformed.

EXAMPLES

[0059] Test Methods

[0060] The following test methods were employed to evaluate thetechnical adhesive properties of the pressure-sensitive adhesivesprepared.

[0061] Shear Strength (Test A)

[0062] A strip of the adhesive tape 13 mm wide was applied to a smoothsteel surface which had been cleaned three times with acetone and oncewith isopropanol. The area of application was 20 mm×13 mm(length×width). The adhesive tape was then pressed onto the steelsupport four times using a 2 kg weight. At room temperature, a 1 kgweight was fastened to the adhesive tape and the time taken for theweight to fall down was measured.

[0063] The shear stability times measured are reported in minutes andcorrespond to the average of three measurements.

[0064] Determination of the Gel Fraction (Test B)

[0065] The carefully dried solvent-free samples of adhesive are weldedinto a pouch of polyethylene web (Tyvek nonwoven). The gel index, i.e.,the toluene-soluble weight fraction of the polymer, is determined fromthe difference in the weights of the samples before and after extractionby toluene.

[0066] 180° Bond Strength Test (Test C)

[0067] A strip 20 mm wide of an acrylate pressure-sensitive adhesive(PSA) was applied to steel plates washed twice with acetone and oncewith isopropanol. The PSA strip was pressed onto the substrate twicewith a 2 kg weight. Immediately thereafter the adhesive tape was peeledfrom the substrate at a speed of 300 mm/min and at an angle of 180°. Allmeasurements were conducted at room temperature under climatizedconditions.

[0068] The results have been reported in N/cm and have been averagedfrom three measurements.

[0069] Production and Crosslinking of the Samples

[0070] Part I: Addition of Esacure KIP 150™

[0071] Preparation of the Reference Polyacrylate

[0072] A 200 L reactor conventional for free-radical polymerizations wascharged with 3.84 kg of acrylic acid, 8.32 kg of N-tert-butylacrylamide,7.84 kg of methyl acrylate, 60 kg of 2-ethylhexyl acrylate and 60 kg ofacetone/isopropanol (96:4). After nitrogen gas had been passed throughthe rector for 45 minutes with stirring the reactor was heated to 58° C.and 40 g of 2,2′-azoisobutyronitrile (AIBN) were added. The externalheating bath was then heated to 75° C. and the reaction was carried outconstantly at this external temperature. After a reaction time of 1 houra further 40 g of AIBN were added. The reaction was terminated after areaction time of 48 hours and the product was cooled to roomtemperature.

[0073] UV Irradiation

[0074] UV irradiation was carried out using a UV unit from Eltosch. Theunit is equipped with a medium-pressure Hg UV lamp having an intensityof 120 W/cm. The swatches were each passed through the unit at a speedof 20 m/min, the specimens being irradiated in a number of passes inorder to increase the irradiation dose.

[0075] The recorded UV dose is reported in J/cm².

Example I/1

[0076] The pressure-sensitive adhesive was blended with 0.5% by weightof Esacure KIP 150™ (from Lamberti) and then concentrated in asingle-screw extruder (from Berstorff). The speed of the screw was 160rpm, and a throughput of 55 kg/h was realized. For concentration, avacuum was applied at 3 different domes. The subatmospheric pressureswere in each case 130 mbar, 75 mbar, and 60 mbar, the lowest vacuumbeing applied in the first dome. The exit temperature of theconcentrated hotmelt was 130° C. For coating, the extrudates were meltedin a slot die (from Pröls). After conditioning at 120° C. for 48 hoursthe melt was coated onto a Saran-primed PET film 23 μm thick. Theapplication rate was 50 g/m². The adhesive tape specimen was thenirradiated with the UV unit, the specific irradiated dose being listedin the table. To determine the efficiency of crosslinking, test method Bwas carried out in each case. To examine the technical adhesiveproperties, test methods A and C were employed.

Example I/1a

[0077] The pressure-sensitive adhesive was blended with 0.5% by weightof Speedcure ITX™ (from Rahn), after which the procedure was as inExample 1. The adhesive tape specimen was irradiated with the UV unit,the specific irradiated dose being listed in the table. For determiningthe efficiency of the crosslinking, test method B was carried out ineach case.

Example I/1b

[0078] The pressure-sensitive adhesive was blended with 0.5% by weightof Irgacure 819™ (from Ciba-Geigy), after which the procedure was as inExample 1. The adhesive tape specimen was irradiated with the UV unit,the specific irradiated dose being listed in the table. For determiningthe efficiency of the crosslinking, test method B was carried out ineach case.

Example I/1c

[0079] The pressure-sensitive adhesive was blended with 0.5% by weightof Irgacure 651™ (from Ciba-Geigy), after which the procedure was as inExample I/1. The adhesive tape specimen was irradiated with the UV unit,the specific irradiated dose being listed in the table. For determiningthe efficiency of the crosslinking, test method B was carried out ineach case.

Example I/1d

[0080] The pressure-sensitive adhesive was blended with 0.5% by weightof Irgacure 184™ (from Ciba-Geigy), after which the procedure was as inExample I/1. The adhesive tape specimen was irradiated with the UV unit,the specific irradiated dose being listed in the table. For determiningthe efficiency of the crosslinking, test method B was carried out ineach case.

Example I/1e

[0081] The pressure-sensitive adhesive was blended with 0.5% by weightof Irgacure 369™ (from Ciba-Geigy), after which the procedure was as inExample I/1. The adhesive tape specimen was irradiated with the UV unit,the specific irradiated dose being listed in the table. For determiningthe efficiency of the crosslinking, test method B was carried out ineach case.

Example I/1f

[0082] The pressure-sensitive adhesive was blended with 0.5% by weightof Speedcure BMDS™ (from Rahn), after which the procedure was as inExample I/1. The adhesive tape specimen was irradiated with the UV unit,the specific irradiated dose being listed in the table. For determiningthe efficiency of the crosslinking, test method B was carried out ineach case.

Example I/2

[0083] A 200 L reactor conventional for free-radical polymerizations wascharged with 4.8 kg of acrylic acid, 3.2 kg of N-tert-butylacrylamide,9.6 kg of methyl acrylate, 62.4 kg of 2-ethylhexyl acrylate and 60 kg ofacetone/isopropanol (96:4). After nitrogen gas had been passed throughthe reactor for 45 minutes with stirring the reactor was heated to 58°C. and 40 g of 2,2′-azoisobutyronitrile (AIBN) were added. The externalheating bath was then heated to 75° C. and the reaction was carried outconstantly at this external temperature. After a reaction time of 1 houra further 40 g of AIBN were added. The reaction was terminated after areaction time of 48 hours and the product was cooled to roomtemperature.

[0084] The pressure-sensitive adhesive was blended with 0.5% by weightof Esacure KIP 150™ (from Lamberti) and then concentrated in asingle-screw extruder (from Berstorff). The speed of the screw was 160rpm, and a throughput of 60 kg/h was realized. For concentration, avacuum was applied at 3 different domes. The subatmospheric pressureswere in each case 160 mbar, 100 mbar, and 65 mbar, the lowest vacuumbeing applied in the first dome. The exit temperature of theconcentrated hotmelt was 130° C. For coating, the extrudates were meltedin a slot die (from Pröls). After conditioning at 120° C. for 48 hoursthe melt was coated onto a Saran-primed PET film 23 μm thick. Theapplication rate was 50 g/m². The adhesive tape specimen was thenirradiated with the UV unit, the specific irradiated dose being listedin the table. To determine the efficiency of crosslinking, test method Bwas carried out in each case. To examine the technical adhesiveproperties, test methods A and C were employed.

Example I/3

[0085] The procedure of Example I/2 was repeated. The polymerization wascarried out using 4.8 kg of acrylic acid, 6.4 kg ofN-tert-butylacrylamide, 12 kg of methyl acrylate, 62.4 kg of2-ethylhexyl acrylate and 60 kg of acetone/isopropanol (96:4).

Example I/4

[0086] The procedure of Example I/2 was repeated. The polymerization wascarried out using 1.2 kg of acrylic acid, 6.4 kg ofN-tert-butylacrylamide, 0.8 kg of maleic anhydride, 35.8 kg of n-butylacrylate, 35.8 kg of 2-ethylhexyl acrylate and 60 kg ofacetone/isopropanol (96:4).

Example I/5

[0087] The procedure of Example I/2 was repeated. The polymerization wascarried out using 0.4 kg of acrylic acid, 6.4 kg ofN-tert-butylacrylamide, 0.8 kg of maleic anhydride, 72.4 kg of2-ethylhexyl acrylate and 60 kg of acetone/isopropanol (96:4).

Example I/6

[0088] The procedure of Example I/2 was repeated. The polymerization wascarried out using 8 kg of acrylic acid, 72 kg of 2-ethylhexyl acrylateand 60 kg of acetone/isopropanol (96:4).

[0089] Part II: Addition of Esacure KIP 150™ and Genomer™

[0090] Preparation of the Polyacrylate

[0091] A 200 L reactor conventional for free-radical polymerizations wascharged with 0.8 kg of acrylic acid, 6.4 kg of N-tert-butylacrylamide,0.8 kg of maleic anhydride, 36 kg of 2-ethylhexyl acrylate, 36 kg ofn-butyl acrylate and 60 kg of acetone/isopropanol (97:3). After nitrogengas had been passed through the reactor for 45 minutes with stirring thereactor was heated to 58° C. and 40 g of 2,2′-azoisobutyronitrile (AIBN)were added. The external heating bath was then heated to 75° C. and thereaction was carried out constantly at this external temperature. Aftera reaction time of 1 hour a further 40 g of AIBN were added. Thereaction was terminated after a reaction time of 48 hours, the productwas cooled to room temperature, and finally 0.8 kg of Genomer 4212™(from Rahn) was added.

[0092] UV Irradiation

[0093] UV irradiation was carried out using a UV unit from Eltosch. Theunit is equipped with a medium-pressure Hg UV lamp having an intensityof 120 W/cm. The swatches were each passed through the unit at a speedof 20 m/min, the specimens being irradiated in a number of passes inorder to increase the irradiation dose.

Example II/1

[0094] The pressure-sensitive adhesive was blended with 30 weightfractions of resin DT 110™ (from DRT) and 0.5 weight fractions ofEsacure KIP 150™ (from Lamberti) and then concentrated in a single-screwextruder from Berstorff. The speed of the screw was 160 rpm, and so athroughput of 60 kg/h was realized. For concentration, a vacuum wasapplied at 3 different domes. The subatmospheric pressures were in eachcase 160 mbar, 70 mbar, and 55 mbar, the lowest vacuum being applied inthe first dome. The exit temperature of the concentrated hotmelt was110° C. Thereafter, using a counterrotatingly operated twin-screwextruder from Welding, 0.3 weight fractions of Genomer 5248™ (fromRahn), based on the polymer, were compounded in at a speed of 320 rpm.For coating, the extrudates were melted in a slot die from Pröls,coating taking place onto a Saran-primed PET film 23 μm thick. Theapplication rate was 100 g/m². The adhesive tape specimen was thenirradiated with 2 and 4 passes through the UV unit. To determine thetechnical adhesive data, test methods A and B were conducted.

Example II/1a

[0095] The pressure-sensitive adhesive was blended with 30 weightfractions of resin DT 110™ (from DRT), 0.5 weight fractions of EsacureKIP 150™ (from Lamberti) and 1.0 weight fractions of Genomer 5248™ (fromRahn). The procedure of Example II/1 was then followed. The mixturegelled in the hotmelt processing operation and could not be used anyfurther.

Example II/1b

[0096] The pressure-sensitive adhesive was blended with 30 weightfractions of resin DT 110™ (from DRT), 0.5 weight fractions of EsacureKIP 150™ (from Lamberti) and the procedure of Example II/1 was thenfollowed.

Example II/1c

[0097] The pressure-sensitive adhesive was blended with 30 weightfractions of resin DT 110™ (from DRT), 0.5 weight fractions of EsacureKIP 150™ (from Lamberti) and 0.5 weight fractions of Genomer 5248™ (fromRahn). The mixture gelled in the hotmelt processing operation and couldnot be used any further.

Example II/2 (Reference)

[0098] The pressure-sensitive adhesive was blended with 30 weightfractions of resin DT 110™ (from DRT) and 0.5 weight fractions ofSpeedcure ITX™ (from Rahn) and then concentrated in a single-screwextruder from Berstorff. The speed of the screw was 160 rpm, and so athroughput of 60 kg/h was realized. For concentration, a vacuum wasapplied at 3 different domes. The subatmospheric pressures were in eachcase 160 mbar, 70 mbar, and 55 mbar, the lowest vacuum being applied inthe first dome. The exit temperature of the concentrated hotmelt wasabout 113° C. For coating, the extrudates were melted in a slot die fromPröls. After conditioning at 140° C. for 1 hour, coating took place ontoa Saran-primed PET film 23 μm thick. The application rate was 100 g/m².The adhesive tape specimen was then irradiated with 2 and 4 passesthrough the UV unit. To determine the technical adhesive data, testmethods A and B were conducted.

Example II/3 (Reference)

[0099] The pressure-sensitive adhesive was blended with 30 weightfractions of resin DT 110™ (from DRT), 0.5 weight fractions of SpeedcureITX™ (from Rahn) and 0.3 weight fractions of Genomer 5248™ (from Rahn).The procedure of Example II/1 was followed.

Example II/4 (Reference)

[0100] The pressure-sensitive adhesive was blended with 30 weightfractions of Resin DT 110™ (from DRT) and 0.5 weight fractions ofIrgacure 819™ (from Ciba-Geigy) and the procedure of Example II/2 wasthen followed.

Example II/5 (Reference)

[0101] The pressure-sensitive adhesive was blended with 30 weightfractions of resin DT 110™ (from DRT), 0.5 weight fractions of Irgacure819™ (from Ciba-Geigy) and 0.3 weight fractions of Genomer 5248™ (fromRahn) and the procedure of Example II/3 was then followed.

Example II/6

[0102] The pressure-sensitive adhesive was blended with 0.5 weightfractions of Esacure KIP 150™ (from Lamberti) and 0.3 weight fractionsof Genomer 5248™ (from Rahn) and the procedure of Example II/3 was thenfollowed.

Example II/7 (Reference)

[0103] The pressure-sensitive adhesive was blended with 0.5 weightfractions of Speedcure ITX™ (from Rahn) and 0.3 weight fractions ofGenomer 5248™ (from Rahn) and the procedure of Example II/3 was thenfollowed.

Example II/8

[0104] The pressure-sensitive adhesive was blended with 25 weightfractions of Piccotex 75™, 0.5 weight fractions of Esacure KIP 150™(from Lamberti) and 0.3 weight fractions of Genomer 5248™ (from Rahn)and the procedure of Example II/3 was then followed.

Example II/9

[0105] The pressure-sensitive adhesive was blended with 25 weightfractions of Piccotex 75™ (from Hercules), 0.5 weight fractions ofEsacure KIP 150™ (from Lamberti) and 0.3 weight fractions of Genomer5275™ (from Rahn) and the procedure of Example II/3 was then followed.

Example II/10

[0106] The pressure-sensitive adhesive was blended with 25 weightfractions of Piccotex 75™ (from Hercules), 0.5 weight fractions ofEsacure KIP 150™ (from Lamberti) and 0.3 weight fractions of Genomer5292™ (from Rahn) and the procedure of Example II/3 was then followed.

[0107] Results

[0108] To demonstrate the efficiency of Esacure KIP 150™ (from Lamberti)various alternative UV photoinitiators were tested in comparison.Irgacure 819™, 651™, and 184™ (from Ciba-Geigy) are standard systems forthe UV crosslinking of polyacrylates. Speedcure ITX™ (from Rahn) isconsidered an extremely reactive photoinitiator and was therefore alsotested in comparison. The results of these investigations are summarizedin Table 1. TABLE 1 UV irradiation; Example number of passes Gel index[%] I/1 0 0 I/1 1 35  I/1 2 74  I/1a 0 0 I/1a 1 9 I/1a 2 26  I/1b 0 0I/1b 1 3 I/1b 2 5 I/1c 0 0 I/1c 1 5 I/1c 2 12  I/1d 0 0 I/1d 1 9 I/1d 216  I/1e 0 0 I/1e 1 1 I/1e 2 3 I/1f 0 0 I/1f 1 14  I/1f 2 30 

[0109] The photoinitiator Esacure KIP 150™ (from Lamberti) (Example I/1)clearly gave the best results. Even at very low doses, very high gelindices were achieved, so that this photoinitiator is very highlysuitable for crosslinking pure polyacrylate PSAs. In comparison to theother photoinitiators, therefore, a much lower technical irradiationexpense (number of UV lamps) is needed, so making the operation muchmore advantageous economically.

[0110] Table 1 also indicates that certain photoinitiators possessvirtually no effect with the crosslinking of the polyacrylates and thatthe difference in terms of the gel indices which can be achieved aftercrosslinking, between crosslinking processes initiated with Esacure KIP150™ and the other systems, is very great. The increase in efficiency istherefore significant.

[0111] It is also known that the low volatility of photoinitiators posesa problem for the hotmelt process. When the oligomeric KIP 150 is usedthis volatility is reduced significantly owing to the high molecularweight; this is a further advantage of the inventive process for hotmeltprocessability.

[0112] To demonstrate the general applicability to acrylate hotmeltcompositions, a number of polyacrylates were prepared, blended withEsacure KIP 150™ and then examined in view of the technical adhesiveproperties particularly for subsequent use as PSA tape.

[0113] The results of the technical adhesive tests for theUV-crosslinked polyacrylates are shown in Table 2. UV irradiation; SSTRT, 10 N BS-steel, Gel Example number of passes [min] [N/cm] index [%]I/1 2 +10000 5.1 74 I/2 2 +10000 4.8 75 I/3 2 +10000 4.6 76 I/4 2   6780 5.2 74 I/5 2    2450 5.6 72 I/6 2 +10000 4.7 68

[0114] The results in Table 2 demonstrate that various pure andsaturated polyacrylate PSAs without further additions can be crosslinkedefficiently using Esacure KIP 150™ For polar acrylate PSAs a very goodcohesion is achieved. The gel indices are about 70% with 2 passes of UVirradiation. In conjunction with the good thermal stability and the lowvolatility as result of the high average molecular weight, a distinctadvantage is produced as compared with conventional photoinitiators.

[0115] In order to demonstrate the efficiency of the combination ofEsacure KIP 150™ (from Lamberti) and the polyamine Genomer 5248™ (fromRahn), a variety of alternative UV photoinitiators were tested incomparison. Irgacure 819™ (from Ciba-Geigy) is the standard system forthe UV crosslinking of polyacrylates. Speedcure ITX™ (from Rahn) isconsidered an extremely reactive photoinitiator and was thereforelikewise tested in comparison. The references were investigated in eachcase with and without accelerator Genomer 5248™ (from Rahn). The resultof these investigations are summarized in Table 1. TABLE 1 Number ofpasses for Get index Example UV irradiation [%] SSZ RT, 10 N [min] II/10 0 19 II/1 2 10  +10000 II/1 4 14  +10000 II/1b 0 0 0 II/1b 2 2 342II/1b 4 6 485 II/2 0 0 34 II/2 2 0 175 II/2 4 2 264 II/3 0 0 15 II/3 2 41478 II/3 4 12  7640 II/4 0 0 28 II/4 2 0 134 II/4 4 0 235 II/5 0 0 18II/5 2 0 127 II/5 4 1 376

[0116] The best results were achieved with the inventive combination ofEsacure KIP 150™ (from Lamberti) and 0.3 weight fractions of Genomer5248™ (from Rahn) as the initiator system. After just 2 UV passes a gelindex of 10% was achieved, which gives the system a high shear strength.Nevertheless, the blend with excessive fractions of amine acceleratorshowed that gelling then occurs by way of the carboxylic acid groups, sothat the synergistic effects of photoinitiator and amine accelerator canbe utilized only with a Genomer 5248™ weight fraction of <0.5 and >0.

[0117] The other two photoinitiator systems (Irgacure 819™ and SpeedcureITX™) are significantly less reactive and require a far higher UVradiation dose. Furthermore, the addition of accelerator substance isessential, since under standard conditions virtually no crosslinkingotherwise occurs.

[0118] The results of the technical adhesive tests for the purepolyacrylates (Examples II/6 and II/7) are shown in Table 2. TABLE 2Number of passes for Example UV irradiation Gel index [%] SSZ RT, 10 N[min] II/6 0 29 1588  II/6 2 76 505 II/6 4 79 448 II/7 0 29 1256  II/7 267 908 II/7 4 76 292

[0119] The results in Table 2 demonstrate that pure polyarylate systemscan also be crosslinked efficiently with the inventivephotoinitiator/accelerator combination. The comparison of Esacure KIP150™ with Speedcure ITX™ also shows a greater efficiency for KIP 150™for the pure polyacrylates. Particularly at a relatively low radiationdose, Esacure KIP 150™ is more reactive than Speedcure ITX™, and this ismanifested in a gel index which increases more rapidly. As far as theshear strength is concerned, the pure polyacrylate systems becomeovercrosslinked with even relatively low irradiation, so that thespecimens irradiated in Table 2 all fail adhesively, and, therefore, thevalues measured are relatively low.

[0120] The results of the technical adhesive tests for Examples II/8 toII/10) are shown in Table 3. TABLE 3 Number of passes for Example UVirradiation Gel index [%] SSZ RT, 10 N [min] II/8 0  0  122 II/8 2 112354 II/8 4 18 6759 II/9 0  0  134 II/9 2 12 2183 II/9 4 20 7140 II/10 0 0  98 II/10 2 13 2542 II/10 4 22 8025

[0121] Examples II/8 to II/10 demonstrate that further resin-blendedpolyacrylate hotmelt PSAs can also be crosslinked efficiently with theinventive combination of accelerator and photoinitiator Esacure KIP 150™without this process being severely adversely affected by the regulatingactivity of the resins during crosslinking. In all cases, UV irradiationproduces a sharp rise in the cohesion of the pressure-sensitiveadhesive, and with 4 passes the values obtained lie within the range foroptimum crosslinking. Moreover, Examples II/8 to II/10 demonstrate thatdifferent accelerators can also be used.

[0122] In summary it can be stated that the inventive processunexpectedly makes available an extremely stable crosslinkable acrylatehotmelt system which has a very high pot life. For saturated systems,therefore, an efficient crosslinking method is available, even in thepresence of additives, resins, and the like. In contrast to systemsprepared by polymer-analogous reactions, there is essentially no gellingreaction.

[0123] Through the addition of (oligomeric) amine acrylates shortlybefore coating it is possible to improve the crosslinking further.

1. A process for preparing a polyacrylate pressure-sensitive adhesive byway of a hotmelt process characterized in that a polyfunctionalα-cleaver present in oligomeric form is added to the polymer to becrosslinked, prior to processing by the hotmelt process, and the UVcrosslinking is carried out after processing by the hotmelt process. 2.The process of claim 1, characterized in that the α-cleaver used is anα-hydroxy ketone, especiallyoligo[2-hydroxy-2-methyl-1-[4-(1-methylvinyl)phenyl]propanone].
 3. Theprocess of at least one of the preceding claims, characterized in thatthe α-cleaver is used at from 0.25 to 1% by weight, based on the polymerto be crosslinked.
 4. The process for at least one of the precedingclaims, characterized in that the polymer to be crosslinked is preparedusing a monomer mixture comprising at least the following components: a)65 to 100% by weight of (meth)acrylic acid and (meth)acrylic acidderivatives of the general formula

where R₁═CH₃ or CH₃ and R₂ is an alkyl chain having 2 to, 20 carbonatoms, b) 0 to 35% by weight of vinyl compounds containing functionalgroups, so that the sum of all of the components used is 100% by weight:5. The process of at least one of the preceding claims, characterized inthat the polymer to be crosslinked has added to, in addition, 0.5 to 40%by weight of one or more resins and/or 0.1 to 0.3% by weight of one ormore amine acrylates-especially oligomeric amine acrylates-based in eachcase on the polymer to be crosslinked.
 6. The process of at least one ofthe preceding claims, characterized in that UV crosslinking is broughtabout by means of brief ultraviolet irradiation within a wavelengthrange from 200 to 400 nm, in particular using high-pressure ormedium-pressure mercury lamps with an output of from 80 to 200 W/cm. 7.The use of a polyfunctional α-cleaver in oligomeric form, in particularan α-hydroxy ketone, especiallyoligo[2-hydroxy-2-methyl-1-[4-(1-methylvinyl)-phenyl]propanone], as UVinitiator for the crosslinking of polyacrylate pressure-sensitiveadhesives passing through the hotmelt process.